Pulse Coral

Image: Richard Ling

When most people think of corals they think of what amounts to an elaborately carved stone resting on the sea floor. It's a pretty stone, no doubt. Brightly coloured, of course. The carving is exquisite and it's probably in some paradisal island, basking unaware in sun-dappled loveliness.

But we know that's scarcely half the story. That lump of stone plays host to potentially thousands of tiny polyps which emerge at night to extend their tentacles and capture prey. Some corals look less like rocks and more like trees or mushrooms. Some have almost none of that stony stuff and are thoroughly soft.

And then there are a few who...

Video: aquarium life

Move!

These are the unique and strangely mesmerising motions of the Pulse Coral.

Dozens of stringy polyps, each topped with 8 feathery tentacles, opening and closing in rhythmic pulsation like a seething mass of grasping mutant hands bouquet of pretty flowers who can't decide whether to open or not. Silly darlings!

Image: Jill Catley

Not all Pulse Corals exhibit this interesting behaviour. There are almost 200 species of them, spread across more than 15 genera comprising the Xeniidae family. They like warm, sunny seas from Africa, through southeast Asia and out to Hawaii.

A lot of them look like mushrooms, with the polyps emerging from the top of a single, thick stalk. Sometimes it's two thick, fleshy stalks instead. In some the stalk divides into lots of slim branches.

Image: Justin Casp

Species of one genus called Anthelia have no stalk at all. They encrust the ground or rocks, instead. Rising from that crust is a miniature forest of those long, thin polyps.

Of the ones that do the pulsing, no-one's quite sure why they do it!

It's certainly not about catching food. Those grasping hands aren't grasping for tiny nibbles in the sea. They scarcely have the structures necessary to digest such prey. Luckily, Pulse Corals are able to rely almost entirely on an abundance of symbiotic, photosynthesising zooxanthellae for their food.

Video: Nervous System

You might wonder if the polyps pulse to allow sunlight to get through to more hard to reach areas of their body. Apparently not, for they don't actually stop when the sun goes down.

Pulse Corals supplement their zooxanthellae's offerings with dissolved nutrients absorbed directly from the sea, and this may take us a step closer to answering this mystery.

Like any other photosynthesising creature, the corals's personal colony of zooxanthellae require a supply of carbon dioxide to do their job. And they create a whole lot of oxygen which has to be gotten rid of, particularly since an overabundance of oxygen reduces their ability to use carbon dioxide.

It's just like you being unable to eat your dinner because of the sickening stench emanating from a toilet that hasn't been flushed in months. Well... it's a little bit like that...

Video: Tidal Gardens Inc.

Thus, the pulsing of the coral creates a turbulent upward flow of water that helps cast out all that stinking oxygen and make way for sweet, delicious carbon dioxide. It also ensures that half the polyps aren't constantly surrounded by oxygen-rich water that has just come from the other polyps. If they were, those poor fellows would have a much tougher time unburdening themselves. And since they're all part of the same colony, that would be bad news for them all.

It's all most effective. One study found that clapping those mutant hands increased photosynthesis ten-fold, more than making up for the energy required to keep pulsing.

Or maybe they're just doing it to be friendly? The family name Xeniidae comes from Xenia, the name of the biggest genus. It comes from a Greek word that refers to the concept of hospitality, like xenophobia but without the phobia.

Does Xenia refer to those beguiling mutant hands beckoning us to join in their strange mutant hand dance? Is it all those zooxanthellae residing in their body?

Video: Eunjae Im

Perhaps it's the crabs and shrimp that can sometimes be seen clambering among the polyps.

Many hands may spoil the broth but what are they like at a back rub?